High intensity discharge (HID) lamps are typically coupled to a ballast which provides an alternating current (AC) signal to the lamps. To ignite the lamps, the ballast must apply a relatively high voltage, 2 kilovolts for example, which initiates an electric arc between the lamp terminals. After ignition of the lamp, the ballast provides operational signal levels for sustaining the electrical arc which is the source for light emitted by the lamp.
One type of ballast known as a magnetic ballast includes a magnetic element, such as a laminated iron core. Magnetic ballasts energize HID lamps with a signal having a frequency at or near the incoming power signal, e.g., 60 Hertz. While magnetic ballasts may provide generally reliable operation, they are relatively inefficient due to the low frequency drive signal. Furthermore, magnetic ballasts incur substantial heat loss which further decreases the operating efficiency.
In an effort to overcome these disadvantages, attempts have been made to replace magnetic ballasts with electronic ballasts. Electronic ballasts receive the low frequency power line signal and provide a relatively high frequency drive signal to the lamp. Due to the high frequency operation of the circuit, electronic ballasts are significantly more efficient than magnetic ballasts.
One problem associated with energizing HID lamps with a high frequency signal occurs when the ballast drives the lamp at a frequency which results in acoustic resonance of the lamp. In general, acoustic resonance refers to arc instability which manifests itself as light flickering. In the extreme, acoustic resonance can generate signal levels which can cause the lamp to explode. The frequency at which acoustic resonance occurs depends upon a number of factors such as the lamp dimensions, gas density, and operating temperature. To avoid acoustic resonance, HID ballasts are typically designed to provide a lamp current which has a frequency greater than about 20 kHz. Despite efforts to achieve a desired operating frequency, over time the ballast may provide a drive signal having a decreased operating frequency that may cause the lamp to enter acoustic resonance. Also, lamp parameter variations due to manufacturing limitations, to changing conditions of the lamp components, and to other contributing factors, can cause the lamp to operate at acoustic resonance.
A further disadvantage associated with electronic ballasts occurs when a lamp fails to ignite. In general, the ballast must apply a relatively high voltage signal, e.g., 2 kV to the HID lamp to initiate an arc. However, for a lamp that fails to ignite, some ballasts generate voltages that may ultimately increase to potentially destructive levels.
It would, therefore, be desirable to provide an HID ballast that minimizes arc instability associated with acoustic resonance. It would also be desirable to provide an HID ballast that avoids applying excessive voltage levels to a lamp that fails to light.